The invention relates to a point absorber wave energy converter, comprising in part an underwater device which derives power from buoyancy variations arising from changes in pressure caused by waves and/or changes in the level on the surface above and which reacts against either the bottom or a second vessel floating on the surface above. The term xe2x80x9cwave motionxe2x80x9d as used herein refers to both waves on a surface of a liquid and swell in a body of a liquid.
Quest for Economic Sources of Renewable Energy
The oil crisis in the early 1970""s was the impetus for much of the pioneering work into wave energy conversion. A lack of practical solutions or reasonable prospects of efficient and robust technologies, plus declining oil prices, led to a general disenchantment. Research continued at a few largely academic centres and over the past twenty-five years a great deal has been learnt. Both theoretical understanding of sea waves and technical expertise in related marine engineering has gained immeasurably from the offshore oil and gas industries during the same period. Growing concern with global climate change has led to an increased sense of urgency in the quest for commercially viable renewable energy sources.
The Size of the Wave Energy Resource
The theoretical potential of wave energy has been recognised for many years. The size of this resource has been estimated to be 219 gigawatts along the coats of the European Union, or more than 180 terawatt hours each year. The wave power off the west coasts of Ireland and Scotland, where the winter resource is approximately twice that available during summer months, ranks with the highest levels per kilometre in the World.
Wave energy is lost by friction with the sea bottom as the sea becomes shallow (water depths of half a wavelength or less). This is most pronounced where wavelengths tend to be long, as off the NW coast of Europe.
Development of Wave Energy Converters
Research and development into wave energy converters (WECs) over the past twenty-five years, plus the knowledge and practical experience gained from the off-shore oil and gas industries, has now reached a stage where robust and effective wave energy converters with installed capacities of one megawatt and greater are being developed.
The wave energy resource may be split into three broad categories, based on where the energy from waves may be recovered:
1. in the open sea, i.e. offshore
2. on or close to the shore line, i.e. on-shore or inshore
3. outside the normal area of breaking waves but not in the deep ocean, i.e. near shore.
A fourth category, not generally considered in the context of wave energy converters, but which may be of relevance to this present invention, is waves or surges in a liquid contained in vessels and tanks.
The very large number of devices and concepts proposed to date has been classified and described in summary form for the Engineering Committee on Oceanic Resources by the Working Group on Wave Energy Conversion (ECOR draft report, April 1998). This follows a similar classification based on the intended location, i.e. off-shore, near shore to off-shore, and on-shore.
Wave Energy Converters (WECs) may also be classified in different ways according to their operating principle and the ways in which they react with waves. In terms of practical application, only a very few types of device are presently, or in the recent past have been, in use or under test in European waters.
By way of illustration, two different but overlapping classes will be briefly commented on: the Oscillating Water Column (OWC) devices and Point Absorbers, the latter being the relevant class in the present context.
OWC devices are typically those where the wave is confined in a vertical tube or a larger chamber and, as it surges back and forth, drives air through a power conversion device. Megawatt-scale OWC devices are now at an advanced stage of development. One such device, being built in a rocky gully on the western shore of Pico in the Azores, is a reinforced concrete chamber partly open at one side to the waves, and with two turbines above and behind through which the confined air is forced. These are specially developed Wells turbines (one with variable blade pitch) and on the whole would seem to be the best-developed and perfected conversion system available today. It is, however, unlikely that any one such installation will have an installed capacity greater than two megawatts and the number of suitable sites has to be extremely limited.
Point absorbers may react against the sealed (therefore necessarily sited near-shore), or be floating and self-reacting. Theoretical analysis has greatly increased our understanding of point absorbers.
Point absorbers are usually axi-symmetric about a vertical axis, and by definition their dimensions are small with respect to the wavelength of the predominant wave. The devices usually operate in a vertical mode, often referred to as xe2x80x98heavexe2x80x99. As such they are capable of absorbing energy arising from changes in the surface level rather than from forward motion of breaking seas. The theoretical limit for the energy that can be absorbed by an isolated, heaving, axi-symmetrical device has been shown to depend on the wavelength of the incident waves rather than the cross section of the device, i.e. from the wavelength divided by 2xcfx80. Thus the wavelength is a critically important criterion, resulting in the attraction of locating the point absorber devices well outside the region of breaking waves, and where they will be open to long wavelength ocean swell or xe2x80x98heavexe2x80x99. A point absorber device may react against the inherent inertia of one of its components, or against the bottom of the sea. Thus, point absorbers may be deployed near-shore in contact with the sea-bed or, in the case of self-reacting devices, near-shore or off-shore.
Small-scale practical point absorbers such as fog horns and navigation buoys, both of which may incorporate OWCs, have been in use for decades. Typically these have a power of a few hundred watts.
One new point absorber device, now claimed to be capable of generating of the order of a megawatt, is described in International Application WO 95/117555. This is based on the buoyancy variations of a submerged, partly air filled, rigid vessel open at the bottom. Initially the device is floating with neutral buoyancy at a certain depth. If a wave passes above it the pressure around this vessel increases and water will flow into the vessel, displacing the air or gas inside (which is free to flow to a large reservoir or to similar devices linked by pipelines), decreasing the air volume in it and hence its buoyancy. The upthrust experienced has decreased in proportion to the volume of water displaced, i.e. Archimedes"" principle. The partially filled vessel will start to sink. When a trough passes above it the reverse process occurs, and the vessel tends to rise to recover its rest position. The size of the forces exerted will depend on the extent of the water surface within the vessel, the amplitude of the wave and the frequency of waves. WO 95/17555 describes the wave energy transformer in terms of two similar containers, horizontally displaced, such that the gas displaced from one container passes to the second. Essentially the gas, being free to move between two or more similar devices remains under constant pressure, as required by the depth below the surface.
International Application WO 95/117555 as described, and its subsequent developments is a heavily engineered device, one that will not readily flex with the lateral movements of water as found below waves, it is not independent of the seabed and is not independent of tidal changes in mean sea level. The base or centre of the device is fixed in its position with respect to the seabed.
A common problem with existing devices designed to harvest significant amounts of energy from the sea is their complexity and cost. They are predominantly large structures, with rigid components, placed in a harsh environment. There is little use of well-proven components. Most devices proposed are very demanding in terms of engineering design, deployment and maintenance.
Other known devices which are used in the marine environment, although not designed for the conversion of wave energy to usable power, include that described in FR-A-2 488 339; which is designed to pump fluids from the sea-bed.
It is an object of this present invention to provide an improved device for extracting energy from sea waves or a swell in a body of liquid.
Accordingly the invention provides: a wave energy conversion apparatus for harnessing energy from wave motion on the surface of a body of liquid comprising:
at least one buoyant member of variable buoyancy for holding a substantially constant mass of gas in an at least partially submerged position in the body of liquid such that as the volume of the substantially constant mass of gas changes with changes of pressure on said buoyant member caused by wave motion a change in buoyancy of the variable buoyant member results; and
a conversion means for converting said change in buoyancy of the buoyant member to an energy output device.
The apparatus preferably comprises further:
a fixed load or non-variable opposing buoyancy connected to the variable buoyant member wherein said load acts to balance the buoyancy of said buoyancy member of variable buoyancy so as to maintain said buoyant member of variable buoyancy in a normal position such that said means for converting said change in buoyancy of the buoyant member of variable buoyancy operates in a double acting manner and
a means whereby the variable buoyancy so loaded may react against the bottom or against a further vessel of non-variable buoyancy floating on the surface.
Preferably any such apparatus may be set up such that it may oscillate in resonance with prevailing wave frequencies.
Preferably said means for converting resulting forces or changes in forces in the apparatus to useful energy may be selected from one or more of the following:
(a) a hydraulic system
(b) a pneumatic system
(c) a mechanical system
(d) a piezo-electric system
Preferably said means for converting said forces or changing forces in the apparatus converts said change to an output device which is selected from one or more of the following:
(a) an electricity generating device
(b) a device for the hydrolysis of water
(c) a pumping device
(d) a device for making potable water
(e) a device for extracting dissolved salts
(f) a hydraulic device
(g) a mechanical device
The buoyant member of variable buoyancy preferably includes means for altering the volume of the buoyant member of variable buoyancy in response to changes in pressure on the buoyant member of variable buoyancy.
Said buoyant member of variable buoyancy may be constructed from a substantially air-tight non-rigid flexible or otherwise elastic material which may be subdivided into two or more compartments.
Alternatively said buoyant member of variable buoyancy may be constructed from a substantially rigid container, substantially open at the bottom, such that the changes in surrounding pressure causes a fluctuation in liquid level within the container. The container or vessel may have a series of apertures on the underside of the container allowing liquid to enter and leave the container, depending on the pressure of the head of liquid on the contained gas. One example would be an inverted bell, with one or more apertures of varying sizes cut into the base.
In one embodiment said load is a mass that may move up or down over a predefined range, and whose position is determined by the buoyancy of said buoyant member of variable buoyancy to which it is linked. The mass and buoyant member are preferably constrained in their movement by a fixed or relatively fixed structure and against which they can react. The fixed or relatively fixed structure may be a mast or vertical spar tethered to the seabed, or a floating vessel of fixed and greater buoyancy that may move in opposition when floating on waves or swell on the surface above.
In a further embodiment said load is a buoyant member of non-variable buoyancy attached to said buoyant member of variable buoyancy by a cable passing a pulley-type structure which acts to balance the buoyancy of the variable buoyancy in its normal condition.
The primary excitation forces arise from the vertical displacement in the waves and corresponding changes in hydrostatic pressures below.
The constant mass of gas within the variable buoyancy responds to changes in surrounding hydrostatic pressures by changes in volume and thus, by Archimedes Principle, experiences changes in upthrust. The volume changes may occur adiabatically or close to adiabatically and are governed by the elementary gas laws.
Power may be taken off by the reaction of the buoyant member of variable buoyancy against the bottom, or a float on the surface moving in opposition, or a submerged reaction plate, or all, or some combination of these.
Other advantages and features of the present invention will become apparent from the following detailed description, the accompanying drawings, and the appended claims.